Chemistry 2e · Essential Ideas

Physical and Chemical Properties

8 min read

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

A property is any characteristic used to describe, identify, or predict the behavior of a substance. Chemists sort properties into two big families. Physical properties can be observed or measured without changing the substance's identity — color, odor, density, melting point, boiling point, hardness, and electrical conductivity are physical. Chemical properties describe how a substance reacts to form new substances — , reactivity with acid or oxygen, and tendency to corrode are chemical. The same distinction applies to changes: a (melting, cutting, dissolving) alters form but not composition, while a (burning, rusting) produces new substances.

Why this matters

Properties are how we tell substances apart and decide what they are good for. Copper is used in wiring because it conducts electricity (); stainless steel is used in surgical instruments because it resists corrosion (); gasoline is stored with fire precautions because it is flammable (chemical property). In health care, drug stability, solubility, and shelf life are all property questions. On exams, the common trap is mislabeling: students call "dissolving" a chemical change or "reactivity" a physical property. This topic gives you a reliable test for every case: Does the identity of the substance change?

The college version

Core Concepts

Physical properties

A physical property is observed or measured without changing what the substance is. Examples: color, odor, density, melting point, boiling point, hardness, malleability (ability to be hammered into sheets), ductility (ability to be drawn into wires), solubility, and electrical conductivity. You can measure the melting point of ice and still have water afterward — identity unchanged. Density, melting point, and boiling point are characteristic "fingerprints" of pure substances.

Intensive versus extensive properties

Physical properties divide further into two kinds. Extensive properties depend on the amount of matter present: mass, volume, and length all grow when you take a bigger sample. Intensive properties do not depend on the amount: temperature, color, density, and melting point are the same for a drop or a barrel of the same substance. Density is especially useful because it is the ratio of two extensive properties (mass/volume), which makes it intensive — a valuable identification tool. Whether a property is intensive or extensive is a frequent exam question, and the test is always the same: cut the sample in half — does the property change?

Chemical properties

A chemical property describes a substance's ability to undergo a chemical change — to react and form new substances. Examples: flammability (ability to burn), reactivity with acids, reactivity with oxygen (rusting or tarnishing), ability to corrode, toxicity, and stability (whether it decomposes over time). Chemical properties are observed only when a chemical change actually occurs; you cannot see that iron rusts just by looking at a nail. Knowing chemical properties is a safety issue: a flammable solvent, a corrosive acid, and a reactive metal each demand different handling.

Physical versus chemical changes

A physical change alters the form or state of matter without changing composition: melting, freezing, boiling, condensing, cutting, crushing, and dissolving are physical. A chemical change (chemical reaction) produces one or more new substances with different properties: burning, rusting, digesting food, baking a cake, and reacting vinegar with baking soda are chemical. Evidence that a chemical change occurred includes a color change, gas bubbles (that are not just boiling), formation of a (solid that falls out of solution), release or absorption of heat or light, and a new odor. No single clue is proof by itself — the definitive test is whether new substances formed.

Identifying substances by properties

Because intensive properties are constant for a pure substance, they identify unknowns: a clear liquid with density 1.00 g/mL, boiling point 100 °C, and freezing point 0 °C is almost certainly water. Forensics, quality control, and clinical labs use property fingerprints to confirm identity and purity.

How It Works / Step-by-Step Process

Identify the substance and list its observable characteristics.
For each characteristic, ask: was the substance's identity changed to observe it? If no, it is physical; if yes, it is chemical.
Decide intensive or extensive: would the property change if the sample were halved?
For a change, ask: are new substances produced? If yes, chemical; if no, physical.
Use intensive properties (density, melting point, boiling point) to identify an unknown.

Common Confusions

Do Not Confuse	With	Difference
Physical property	Chemical property	Physical is observed without changing identity (melting point); chemical requires a reaction (flammability).
Physical change	Chemical change	Composition unchanged (melting) versus new substances formed (burning).
Melting	Dissolving	Melting is a phase change of a pure substance (ice → water); dissolving mixes solute into solvent, forming a mixture.
Boiling	Chemical gas evolution	Boiling is physical vaporization of the liquid itself; gas bubbles from a reaction (vinegar + baking soda → CO₂) signal a chemical change.
Intensive property	Extensive property	Intensive is constant per unit amount (density); extensive scales with amount (mass).
Color change as proof of reaction	Color change as always chemical	Mixing food coloring into water changes color but is physical; a permanent new color from a reaction is chemical.
"Density of water is 1 g/mL" as universal	Density varying with temperature	Water's density is about 1.000 g/mL near 4 °C and about 0.998 g/mL at 20 °C — it changes slightly with temperature.

Eli explains

The same idea, in plain words

Explain it like I’m 10

A property is a way to describe stuff: "red," "heavy," "melts easily," or "burns fast." Physical properties you can check without changing what the thing is — melting an ice cube still leaves you with water. Chemical properties only show up when the thing changes into something new — wood burning turns into ash and smoke. Some properties depend on how much you have (mass), but others like density stay the same whether you have a drop or a swimming pool.

Worked examples

A liquid sample has a mass of 52.5 g and a volume of 50.0 mL. Its density is:

$ρ = m V = 52.5 g 50.0 mL = 1.05 g/mL$

Now pour out half the sample: 25.0 mL with a mass of 26.25 g. The new density:

$ρ = 26.25 g 25.0 mL = 1.05 g/mL$

Mass halved (extensive), but density stayed 1.05 g/mL (intensive). This is why density identifies substances regardless of sample size, while mass alone cannot.

Consider two ways water changes. (a) Water boils in a covered pot and condenses back to liquid: the vapor is still H₂O, the mass of captured water is unchanged, and no new substance formed — a physical change (vaporization, then condensation). (b) Water is electrolyzed, and two different gases bubble off: hydrogen (which pops when ignited) and oxygen (which relights a glowing splint). The gases have new properties, so a chemical change occurred — water molecules were split into H₂ and O₂. The same starting material, but the composition test separates the two cases: physical change kept H₂O intact; chemical change made new substances.

Key takeaways

Physical property: observed without changing identity (color, density, melting point, conductivity).
Chemical property: observed only through a chemical change (flammability, reactivity, corrosiveness).
Intensive properties (density, temperature) do not depend on amount; extensive properties (mass, volume) do.
Physical change: same substance, new form. Chemical change: new substances.
Evidence of chemical change: color change, gas, precipitate, heat/light, new odor — but confirm new substances formed.
Density ρ= m/V is intensive and is a powerful identification tool.
Safety: chemical properties like flammability and toxicity determine how a substance must be handled.

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Classify each as physical or chemical property: melting point, flammability, density, reactivity with acid, electrical conductivity, tendency to tarnish.
Show answer
Physical: melting point, density, electrical conductivity. Chemical: flammability, reactivity with acid, tendency to tarnish.
A 20.0 g sample of a liquid occupies 25.0 mL. Compute its density, and state whether density is intensive or extensive.
Show answer
ρ= 20.0 g / 25.0 mL = 0.800 g/mL. Density is intensive — it does not depend on sample size.
Explain why dissolving sugar in water is a physical change, but digesting it in the body is a chemical change.
Show answer
Dissolving is physical: the sugar molecules stay sugar molecules, and evaporating the water recovers them. Digestion is chemical: enzymes break sugar molecules into new substances (CO₂, water, and energy carriers) with different properties.
List three pieces of evidence that a chemical change occurred, and for each, name a physical process that could mimic it.
Show answer
Color change (mimicked by mixing dyes), gas bubbles (mimicked by boiling), heat released (mimicked by friction). The definitive test is whether new substances formed, not the clue alone.
Why can density be used to help identify a substance, while mass alone cannot?
Show answer
Mass is extensive — any amount can have any mass. Density is intensive and characteristic of the substance, so it narrows the identity.
Rusting of iron requires oxygen and moisture. Is "rusts easily" a physical or chemical property? Explain.
Show answer
Chemical. It describes iron's ability to react with oxygen and water to form a new substance, rust; you only observe it when the reaction occurs.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsKey vocabulary · Related

Key vocabulary

physical property: A characteristic observed or measured without changing the substance's identity.
chemical property: A characteristic describing how a substance reacts to form new substances.
intensive property: A property independent of the amount of sample (density, temperature, color).
extensive property: A property that depends on the amount of sample (mass, volume, length).
physical change: A change in form or state with no change in composition.
chemical change: A process that produces new substances with different properties.
flammability: The ability of a substance to burn in the presence of oxygen.
precipitate: A solid that forms and falls out of solution during a reaction.

Sources & references

openstax.org — Chemistry 2e

This lesson was adapted from the open educational references above; their licenses and attributions are preserved. See Copyright & Licensing.

Educational content only. It is not medical, legal or professional advice. Found an error? Tell us.